It is known to produce various metal articles by a technique generally known as squeeze casting in which molten casting metal, usually aluminum or aluminum alloy material, is introduced into a die and a great amount of pressure exerted on the metal as it solidifies to essentially forge or squeeze-form the article in the die producing a metal article having lower porosity and improved mechanical properties as compared to the same article produced by more conventional lower pressure casting methods.
Difficulties have been encountered, however, in trying to squeeze cast articles having a configuration with re-entrant cavities or internal passages necessitating the use of a core. Because of the high pressures involved, conventional sand coring may not be used since the molten casting metal would penetrate the sand core. Similar difficulties are encountered with soluble salt cores. Salt cores are further susceptible to stress cracking during high pressure casting.
U.S. Pat. No. 4,712,600 proposes utilizing a melt-out metal core to produce an insulating air chamber and oil passage within a piston. The low melting point core metal is completely covered with a protective porous member. The molten casting metal penetrates the porous member causing the porous member to be embedded in the resultant article and further encapsulating the melt-out core within the article. To remove the core, a hole is drilled through the article to gain access to the core at which time the article and core may be heated and the core extracted from the article through the drilled hole. The porous member adds cost and complexity to the making of the article. Its incorporation into the article may not be permissible in some applications and could negatively affect the physical and mechanical properties of the cast article.